Polymer-bioactive agent conjugates containing a bioactive agent covalently bound to a polymer are of interest for the targeted and controlled delivery of therapeutic agents. In the treatment of many different conditions, the site-specific delivery of a drug directly to or near a desired site of action in the body of a subject can be highly desirable to improve the efficacy and/or safety of the drug. Certain sites in a subject may require sophisticated delivery vehicles to overcome barriers for effective drug delivery. For example, the eye has a limited volume for administration and requires a pharmaceutical product with a high drug loading to ensure that adequate doses of drug can be delivered while keeping product volume to a minimum. Despite the limited volume it is desirable to be able to deliver drug to the site continuously and in a controlled manner over an extended period of time.
β-blockers are antagonists of beta-adrenoreceptor sites and are used to treat or manage a range of conditions, including cardiac arrhythmias, hypertension, hypotension and glaucoma. Elevated intraocular pressure (ocular hypertension) is a risk factor for glaucoma. β-blockers can reduce intraocular pressure and exert an ocular hypotensive effect by reducing the production of aqueous humour in the eye.
Prostaglandin analogues are molecules designed to bind to a prostaglandin receptor and are used to treat gastro-intestinal acid related disorders such as duodenal and gastric ulcers, as abortifacients or uterotonics to induce labour or prevent past partum haemorrhage, and to treat ocular hypertension. Prostaglandin analogues exert an ocular hypotensive effect by increasing uveoscleral outflow of aqueous humour.
Prostaglandin analogues and β-blockers used in the treatment of glaucoma are presently formulated as eye drops, which if administered conscientiously to the affected eye will lower intraocular pressure. This in turn can slow the progression of glaucoma. The prostaglandin analogues and β-blockers are administered as eye drops, either alone (i.e. as a single agent) or in combination. It is postulated that combining prostaglandin analogues with β-blockers that exert their effect through a different mechanism, may provide an additive effect in reducing intraocular pressure. For example, some pharmaceutical preparations used in the treatment of glaucoma, such as Xalacom™ eye drops marketed by Pfizer and Ganfort™ eye drops marketed by Allergan, contain a prostaglandin analogue in combination with a β-blocker.
Unfortunately, as glaucoma is an asymptomatic disease many patients do not use their drops conscientiously, compromising therapy. A recent study by Friedman et al. (Friedman et al. IOVS 2007:48, 5052-5057) showed that adherence to glaucoma treatment options is poor with only 59% of patients in possession of an ocular hypotensive agent at 12 months, and only 10% of patients used such medication continuously. Patient compliance in glaucoma therapy is therefore an issue.
Drug delivery systems have been developed to aid in the administration and/or sustained delivery of bioactive agents (such as drugs) to a desired site of action. One mode of delivering a drug to a subject involves the use of a polymer in association with the drug so that it can be delivered to and/or retained at a specific location.
One form of a polymer/drug delivery system utilises an admixture of a polymer with a drug, where the drug is blended with the polymer matrix. However, such admixtures generally result in poor control over the release of the drug, with a “burst effect” often occurring immediately after administration and significant changes in the physical properties of the admixture occurring as the drug is released (Sjoquist, B.; Basu, S.; Byding, P.; Bergh, K.; Stjernschantz, J. Drug Metab. Dispos. 1998, 26, 745). In addition, such admixtures have limited dose loading capacity, resulting in a prohibitively large device for convenient administration to some sites in a subject.
Another form of a polymer/drug delivery system is based on the polymerisation of a drug so as to incorporate the drug molecule as part of the backbone of a polymer chain. Such a system is described in U.S. Pat. No. 6,613,807, WO2008/128193, WO94/04593 and U.S. Pat. No. 7,122,615. However, such polymer systems generally provide inefficient delivery of the drug, as release of the drug relies on breakdown of the polymer backbone. Furthermore, breakdown of the polymer backbone produces inactive intermediates. Such intermediates can complicate regulatory approval, which may require the safety of the intermediates to be demonstrated.
Another approach for preparing polymer-bioactive agent conjugates involves the covalent attachment of bioactive agent molecules to a pre-formed polymer backbone. Examples of such polymer conjugates have been reviewed in Nature Reviews: Drug Discovery 2003:2, 347-360. However, this approach can also be problematic. In particular, steric and thermodynamic constraints can affect the amount of bioactive agent that can be covalently attached, and also impact on the distribution of the bioactive agent along the polymer backbone. These factors can, in turn, reduce control over the release of the bioactive agent. Furthermore, the use of a pre-formed polymer backbone provides limited scope for modification of the polymer conjugate after attachment of the bioactive agent should the properties of the conjugate need to be adjusted to improve drug release and/or to aid patient comfort, particularly in the eye.
For efficient delivery, bioactive agents such as drugs are ideally pendant from the backbone polymer chain.
In preparing polymer-bioactive agent conjugates, step-growth polymerisation is one approach that has been used. By means of step-growth polymerisation, polymer-bioactive agent conjugates can be prepared by covalently reacting a bioactive agent-functionalised monomer having at least two terminal reactive functional groups, with a co-monomer of complementary terminal functionality. An example is the reaction of a drug-functionalised dihydroxy monomer with a diisocyanate co-monomer to form a polymer-drug conjugate with a polyurethane polymer backbone. However, one problem with step-growth polymerisation methods is that many bioactive agents, such as drug molecules, can contain multiple functional groups that are capable of participating in the covalent reactions used to form the polymer. In such circumstances, there is a risk that a functional group on a drug molecule could react with a terminal functional group of a monomer, leading to intra-chain incorporation of the bioactive agent in the polymer. As a result, the bioactive agent becomes part of the polymer backbone structure, rather than forming a pendant group. Prostaglandin analogues and β-blockers are examples of such drugs with multiple nucleophilic functional groups with a consequential high risk of in-chain incorporation.
It would be desirable to provide new polymer-bioactive agent conjugates, which address or ameliorate one or more disadvantages or shortcomings associated with existing materials and/or their method of manufacture, or to at least provide a useful alternative to such materials and their method of manufacture.